It is proposed that the concentration of carboxyl groups should be reduced by using a sealant for carboxyl groups since the hydrolysis of a compound having an ester bond such as a polyester is promoted by a polar group such as a carboxyl group (Patent Document 1, Patent Document 2). A carbodiimide compound is used as the sealant for carboxyl groups.
However, as the carbodiimide compound is a linear compound, foul odors are produced from a volatile isocyanate compound which is formed as a by-product when the carbodiimide compound is used, thereby worsening the work environment.
To cope with this, the applicant found a cyclic carbodiimide compound as a sealant which does not form an isocyanate compound as a by-product even when it reacts with a carboxyl group and filed an international patent application for this (Patent Document 3). However, an industrial process for the production of this useful cyclic carbodiimide compound is not established yet.    (Patent Document 1) JP-A 2004-332166    (Patent Document 2) JP-A 2005-350829    (Patent Document 3) PCT/JP2009/071190